The present invention relates to an image reading device for optically scanning a document surface to thereby output image data and more particularly to an image reading device for scanning a document being conveyed along a glass platen at a constant speed.
Image reading devices are generally classified into two types, i.e., a scanning type device including movable optics for illuminating a stationary document and a sheet-through type device including stationary optics for illuminating a moving document. Also, there are available an ordinary, analog image reading device and a digital image reading device using a CCD (Charge Coupled Device) image sensor or similar image sensor.
Today, a digital image reading system of the type moving a document over stationary optics is attracting attention because it can read even large-size documents and facilitates various kinds of image processing to follow. This type of image reading device, however, has a problem that the slit exposure range (reading exposure width hereinafter) is narrow. For example, if the reading density is 400 dpi (dots per inch), then the reading exposure width is as narrow as 25.4/400=0.0635 mm. Therefore, dust sized 0.0635 mm or above existing in the above range is read as false black data along with true data without regard to an image existing on a document. When a document is read in the presence of such dust, the dust appears in the resulting image in the form of a black stripe or similar defect.
An analog image reading system, whether it is of the scanning type or of the sheet-through type, illuminates a document over a slit width of 5 mm to 10 mm. In such a slit width, a latent image is not formed on a photoconductive element if the photoconductive element and a document are not moved in synchronism with each other. Therefore, dust deposited on a glass platen simply appears in an image as a black spot, which corresponds in size to the dust, in the case of the scanning system or as a blurred latent image in the case of the sheet-through system. The latent image is not conspicuous when developed.
Japanese Patent Laid-Open Publication No. 8-123157, for example, discloses an image reading device in which a document is conveyed while being spaced above a glass platen in order to prevent dust from depositing or staying in an illuminating section. Even this kind of image reading device cannot fully obviate the deposition of dust on the glass platen because the leading edge and trailing edge of a document contact the glass platen.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-197566, 11-136436 and 2000-50023.